Hypophosphatemic Osteomalacia in a Young Adult

Tumor-induced osteomalacia (TIO), otherwise known as oncogenic osteomalacia, is a rare paraneoplastic syndrome, characterized by hypophosphatemia due to decreased tubular reabsorption and low or inappropriately normal level of active vitamin D. The syndrome, first recognized by Robert McCance in 1947, is well described in the medical literature. However, the diagnosis can be delayed due to the nonspecific nature of its presentation. The tumor responsible for TIO produces fibroblast growth factor 23 (FGF-23) which plays a role in regulating renal handling of phosphate and 25-hydroxyvitamin D 1 alpha-hydroxylase activity. Chronic hypophosphatemia eventually leads to inadequate bone mineralization and osteomalacia. The diagnosis should be considered when a patient presents with low phosphate and osteomalacia or rickets and should be differentiated from other disorders of phosphate metabolism such as X-linked, autosomal dominant and recessive hypophosphatemic rickets, and acquired cause like vitamin D deficiency. The localization of the tumor is rather difficult as the tumor can be too small and be anywhere in the body. A combination of thorough physical examination, laboratory tests, and proper imaging is needed for the diagnosis. Surgical removal of the tumor often leads to complete resolution of the syndrome. If the tumor is undetectable or unresectable, then phosphate and vitamin D supplements should be considered.